FIG. 12(a) shows a vertical cross sectional view of a conventional piezoelectric transformer unit, and FIG. 12(b) shows a horizontal cross sectional view of the unit. As shown in the figure, the transformer unit comprises a piezoelectric transformer element 11, an upper case 12a, a lower case 12b, upper lead terminals 13a, lower lead terminals 13b, and projections 14.
The upper and lower cases 12a and 12b are made of molded resin, and the lead terminals 13a and 13b are insert-molded into the cases 12a and 12b. Specifically, three upper lead terminals 13a are insert-molded in the upper case 12a, and three lower lead terminals 13b are insert molded in the lower case 12b.
As shown in FIG. 12(b), the piezoelectric transformer element 11 has three electrodes 11a formed on its lower surface. Also, although not shown in the figure, the element 11 has three similarly disposed electrodes 11a formed on its upper surface. The lower lead terminals 13b respectively contact oscillation nodal points on the electrodes 11a on the lower surface of the element 11, and the upper lead terminals 13a respectively contact oscillation nodal points on the electrodes 11a on the upper surface of the element 11. Thus, the lead terminals 13a and 13b support the piezoelectric transformer element 11 within the casing 12a and 12b as shown in FIG. 12(a).
Also, the projections 14 are formed at various locations on the inner surfaces of the casing 12a and 12b and protrude towards the interior of the casing 12a and 12b. The purpose of the projections 14 is to prevent the piezoelectric transformer element 11 from excessively shifting from a predetermined position with respect to the lead terminals 13a and 13b.
In the conventional device described above, the piezoelectric transformer element 11 is only supported by the elasticity of the lead terminals 13a and 13b at the upper and lower surfaces of the element 11. As a result, any jolt or impact to the device may easily cause the piezoelectric transformer element 11 to shift horizontally with respect to the lead terminals 13a and 13b and abut against the projections 14 and/or inner wall of the casing 12a and 12b. When the element 11 touches the projection 14 or casing 12a and 12b, the oscillation characteristics of the element 11 are deteriorated. In addition, when the element 11 abuts against the projections 14 or casing 12a and 12b, an oscillation having a frequency in the audible range is added to the piezoelectric transformer element 11, and such oscillation is propagated to the casing 12a and 12b and causes noise.